1. Field of the Invention
This invention relates generally to an electronic flash having a light integrator circuit which provides an anticipated quench, and more particularly, to an electronic flash having a light integrator circuit which enables the quench to be anticipated by a predetermined time in order to avoid overexposure under conditions where the photographic subjects are close to the camera or cases of relatively high scene light reflectance.
2. Description of the Prior Art
Circuits for automatically controlling and terminating the operation of a light flash source are well known in the art. Such circuits are found to have particular application in the photographic fields where they are used to control the period of time for which an electronic photo flash lamp is operative. The electronic flash lamp controlling circuits generally include a photosensitive or photoresponsive element located close to its associated camera and are operative to initiate operation of the electronic flash lamp when the camera shutter is opened, and to terminate operation of the electronic flash lamp when a desired total amount of light from the subject has been received by the photoresponsive device. Electronic flash controlling circuits of the prior art have, for the most part, used some kind of light integrating technique to derive an electrical signal representative of the total light received by the photosensitive device over the time period of interest. One type of light integrating circuit which is of particular interest because of its highly linear output signal is disclosed in U.S. Pat. No. 3,620,143, entitled "Automatic Exposure Control System with Fast Linear Response", by J. Burgarella, issued Nov. 16, 1971, in common assignment herewith.
Many of the prior art electronic flash control circuits have not provided control which is sufficiently accurate over a broad range of camera-to-subject distances. In particular, such control circuits have been particularly deficient in providing the desired flash control response for photographic subjects located at a relatively short distance from the camera or in cases of relatively high scene light reflectance.
One such arrangement for providing a fast response under conditions of close camera-to-subject distances is disclosed in U.S. Pat. No. 3,875,471, entitled "Photo Flash Source Control Circuit", by R. Buck, issued Apr. 1, 1975, which shows a light integrating control circuit having a programmed current source for varying the reference voltage at which a comparator is triggered by the output signal from the scene light integration circuit. Thus, under conditions of close camera-to-subject distances the programmed current source provides a relatively low reference voltage to the comparator so that the comparator is triggered earlier by the output signal from the scene light integrating circuit to provide a fast response. One disadvantage of this arrangement is that the programmed current source must be turned on in exact synchronism with the triggering of the flash tube in order that the variable reference voltage be applied to the comparator in a consistent manner for different exposures.
Another arrangement for avoiding overexposure which often occurs with conventional electronic flash apparatus when taking close-up photographs is disclosed in U.S. Pat. No. 3,896,642, entitled "Control Circuit for Electronic Flash Apparatus", by M. Sabanci, issued Mar. 4, 1975, which shows a capacitor, a resistor, and an inductance dimensioned so that the time constant formed from the resistor and capacitor equals the time constant formed by the resistor and inductance. This enables the response time of the control circuit to be measured in nanoseconds rather than in microseconds. However, this arrangement as well as other similar arrangements are only suitable for use with more conventional light integration circuits of the type having a serially connected photoresponsive element and light integrating capacitor and are not readily adaptable for use with the linear light integration circuit as described in the aforementioned U.S. Pat. No. 3,620,143.
Therefore, it is a primary object of this invention to provide a light integration circuit having a highly linear output response under normal ambient light conditions, which is also suitable for use in an electronic flash control circuit to provide the required fast output response with a predetermined anticipation time to correct for the reaction time of the strobe quenching circuit under conditions of close camera-to-subject distances or high scene light reflectance.
It is also an object of this invention to provide an electronic flash of the quench type having a highly linear light integration circuit for satisfactorily controlling the quench under conditions of close camera-to-subject distance or high scene light reflectance.
Other objects of the invention will be in part obvious and will in part appear hereinafter. The invention accordingly comprises a circuit and system possessing the construction, combination of elements and arrangements of parts which are exemplified in the following detailed disclosure.